Magnetoelastic force transducers are previously known from, for example, Swedish Patent No. 151 267. Such a transducer comprises a force-subjected magnetic core built up of magnetoelastic sheets, which core is provided with four through-holes for two windings located perpendicular one to the other, one winding being an excitation winding adapted to be connected to an a.c. source for generating a magnetic flux in the core, and the other winding being a measuring winding for sensing the magnitude of the magnetic flux passing through the measuring winding. The magnitude of this flux depends on the magnitude of the external force applied to the core.
Transducers of the above-mentioned type often suffer from linearity and zero voltage problems. The linearity can be improved by constructing the transducer with two measuring portions, for example in the manner described in U.S. Pat. No. 4,193,294, one portion being substantially subjected to tensile stress and the other being subjected to compressive stress, both stresses being proportional to the external force. A good linearity can then be attained by summing the respective signals from the measuring portions in suitable proportions.
The zero voltage of a magnetoelastic transducer is the phenomenon of a finite output voltage when there is no input force and is caused by internal stresses and crystal orientation in the sheet material of the core, which have arisen, inter alia, during the manufacturing processes of the sheet material, for example during rolling.
Published European Patent Application No. 0,089,916 shows an embodiment of a magnetoelastic transducer with a specially formed sheet section with a measuring part formed as a doubly-bent beam. This transducer exhibits extremely good linearity and low zero voltage values.
In many applications nowadays it is a requirement that the output signal of a force transducer should be zero at zero applied force. Also, for safety reasons it is often a requirement that a residual zero voltage must not be electrically compensated for, since in the event of an electricity supply failing, offsetting a zero voltage may have serious consequences.
In connection with the manufacture of any device and thus a magnetoelastic transducer, it is greatly to be desired that the device should be simple to manufacture and should not require too high a degree of precision.
According to the invention described in the U.S. patent application Ser. No. 130,545, filed Dec. 9, 1987 the transducer has a core of magnetic material made from two cylindrical and identical bodies which are held together with the aid of a stud bolt. In an axially and concentrically inner space there is located a bobbin supporting two measuring windings and one excitation winding adapted to be supplied with alternating current. The interior of the transducer core is formed such that, adjacent to the respective measuring winding in each one of the bodies, there is provided a thin cylindrical tubular wall of magnetic material. When the transducer core is influenced by an external force via the stud bolt, a compressive stress arises in one of these tubular walls and a tensile stress arises in the other tubular wall. In a known manner these stresses influence the magnetic conditions in the core in such a way that a signal, proportional to the applied force, is obtained from the measuring windings which are electrically connected together in opposition.
Excellent though the transducer described above is in many respects, it does have a limited performance as regards its ability to generate a zero signal at zero applied force. With the arrangement described in the aforesaid copending U.S. patent application therefore, the feasible fields of application are limited. Since the design is simple and is favourable from the point of view of ease of manufacture, it would be desirable to be able to improve the zero voltage performance in some way. The present invention relates to a design which corresponds broadly to the disclosure of the aforesaid copending U.S. patent application but which has a considerably improved zero voltage performance.